"We see this as a credible first step in the eventual release of all the Chibok girls in captivity," Nigerian information minister Alhaji Lai Mohammed told media in Abuja.

“We welcome reports ... of the negotiated release of 21 of our abducted #ChibokGirls today," the Nigeria-based #Chibokgirls campaign said in a statement following the release. "Following this development, we trust that our government will continue to work to keep the safety, security, and well-being of the other girls a high priority.”

"We further urge the international community to continue to support our government's effort to rescue all other abducted Nigerians, so that parents, the Chibok community, the nation, and the world can finally put an end to this nightmare once and for all."

In April 2014 the kidnapping of the 276 school girls aged between 16 to 18 caused global outrage and sparked the international #BringBackOUrGirls campaign. While 57 managed to escape almost immediately, around 200 have remained until now.

Negotiations are expected to continue for the release of the remaining girls.